1. Field Of The Invention
The invention relates to a linear actuator having an emergency lowering mechanism.
2. The Prior Art
Briefly, a linear actuator consists of a spindle driven by a reversible electric motor via a transmission. A nut is provided on the spindle, to which an driving rod is secured. The spindle nut is secured against rotation and moves outwards or inwards along the spindle depending on the direction of rotation of the motor and the spindle, and the driving rod is either extended or retracted with the nut.
Depending on the structure in which the actuator is incorporated, the actuator may be equipped with a quick release function or an emergency lowering function. Quick release is typically used in actuators for hospital or nursing beds, e.g., in order to be able to lower the backrest instantaneously in the event that the patient on the bed has a heart attack or goes into a coma. Emergency lowering is typically used for actuators which are incorporated in patient lifters in order to be able to lower the patient because of a power failure. Here, it is a matter of lowering the patient as gently as possible, while with the quick release it is a matter of acting as quickly as possible in order to save lives.
The present invention relates to emergency lowering, and examples of this are known, e.g., from DE 196 47 556 A1 to Rudolf Weiner, where the driving rod is released by disconnecting a pin, whereby the driving rod when being gripped may be turned inwards by hand. In the actuator disclosed in DK 130 460 to Trioterm A/S, the driving rod is secured by a clamping bushing, and by loosening this with an operating grip, the driving rod is released. DE 199 50 689 A1 to Okin discloses a structure with two coupling parts which are spring biased for mutual engagement. When the coupling is released, the driving rod may be turned manually inwards. EP 0 789 665 (U.S. Pat. No. 5,836,462) discloses a structure with a biased friction disc where the friction is adapted such that the driving rod is secured against rotation in normal operation, but may be overcome with an outer torque, viz. by rotating a bushing, whereby the driving rod is screwed back. Finally, EP 1 457 710 A1 discloses a structure comprising a claw coupling enclosed by a screw spring which serves as a brake. Turning of the operating grip through a certain angle causes the claw coupling to be released, and the load on the mounting bracket hereby causes the spindle nut and the driving rod connected with the nut to be screwed inwards on the non-self-locking spindle, whereby the claw coupling again connects the brake spring and thereby brakes the further movement of the driving rod. In order to lower the load, the claw coupling must thus be operated several times.
The object of the invention is to provide a simplified, but reliably operating structure of an emergency lowering device.